This invention relates to manufacture of semiconductor devices, and more particularly to a method of achieving phase shifting performance in high resolution microlithography.
Because of the higher level of integration in the design of recent DRAM (dynamic random access memory) chips, the required structure size of elements within the DRAM the circuit is decreasing. For example, a typical 4-Mbit DRAM requires 0.65 micron feature size, a 16-Mbit DRAM requires 0.5 micron and 64-Mbit requires 0.35 micron. The achievable feature size R projected on the silicon wafer by the conversion lens used in microlithography is limited by diffraction effects and is proportional to the wavelength L of the light source used, and a manufacturing parameter K, while it is inversely proportional to the numerical aperture NA of the lens, i.e. EQU R=KL/NA
So, there are three ways to decrease the structure size: (1) to use shorter wavelength light source, (2) to use higher NA lens, and (3) to improve the manufacturing parameter. The wavelength can be made shorter, for instance, by using an excimer laser with a wavelength of 248 mm, as compared to 365 nm which is the wavelength of the most widely used mercury lamp. High NA optical systems are under development. The phase shifting method improves the manufacturing parameter. The K value of the conventional method using a transparent mask has a theoretical lower limit of about 0.5. By using a phase shifting mask, however, it is possible to improve K down to 0.25. That means, by using the phase shift mask the resolution at the wafer can be improved by 100%, i.e. the minimal feature size can be half of that produced by the conventional method using the same wavelength and NA. Thus, the phase shifting technique has the potential to contribute to the production of higher density 256 Mbit and 1 Gbit DRAMs in the future.
Phase-shift masking is described by Levenson, "What is a Phase-Shifting Mask," SPIE Vol. 1496 10th Annual Symposium on Microlithography (1990), pp. 20-26, by Burggraff, "Lithograpy's Leading Edge, Part 1: Phase-shift Technology," Semiconductor International, February 1992, pp. 42-47, by Schellenburg et at, "Real and Imaginary Phase-Shifting Masks," BACUS News-Photomask, January 1993, pp. 1-12, and in U.S. Pat. No. 4,890,309 issued to Smith et al.
There are problems associated with state-of-the-art phase shifting masks as used in photolithography for semiconductor manufacture. These problems of the prior phase shifting mask technology are associated with the optical delay elements in the beam path. It is difficult to make phase shifting elements on the mask because very high geometrical precision is needed. It is also very difficult to achieve the exact thickness of the phase shifting elements that is required for the desired amount of phase shift. Furthermore, it is impossible to adjust the thickness of the phase shifting element once it is made. As a result of this the manufacturing cost of the masks is very high, which is one of the main problems in introducing phase shifting techniques into practical production lines. In addition, the damage threshold of the phase shifting element is relatively low compared to the substrate or other elements of the mask. This results in lower allowable exposure intensities, which in turn may limit the throughput of the whole production line. All of these problems become especially serious when the wavelength of the illuminating light is shortened.